A source involved in the investigation said the motorist carjacked by Dzhokhar Tsarnaev, right, and his older brother Tamerlan Tsarnaev stated the men planned to drive to New York before the shootout in Watertown. Photo Credit: AP/The Lowell Sun; AFP/VKontakte

BOSTON -- The twin bombs that exploded near the finish line of the Boston Marathon were probably detonated with long-range remote controls of the kind found in children's toys, according to an "intelligence bulletin" sent to police agencies.

The bulletin was issued Wednesday by the FBI and the U.S. Department of Homeland Security to warn law enforcement agencies of the potential threat of such toys. It was shared with Newsday by a federal law enforcement source involved in the bombing investigation.

Federal authorities have accused two men, Dzhokhar Tsarnaev, 19, and his brother, Tamerlan, 26, of placing backpacks containing the bombs at the Boston Marathon finish line and detonating them after walking a safe distance away. Three people were killed and at least 260 were injured.

The bombers' possible use of remote control devices and the design of the pressure-cooker bombs -- they were packed with nails and BBs -- suggests that the planning of the attack was more sophisticated than investigators had initially thought, the source said.

Reconstructing bomb site

Investigators drew these conclusions based on initial reconstructions of the bombs from debris recovered from the blast sites, the source said.

The bulletin states that: "Based on preliminary analysis of recovered evidence, each device likely incorporated an electrical fusing system using components from remote control toy cars such as a transmitter and receiver pair operating at 2.4 GHz, an electronic speed control used as the switch mechanism and sub-C rechargeable battery packs at the power source."

A similar device was found in the failed May 2010 bombing of Times Square, the source said.

Investigators in the Boston Marathon case believe the bombers were motivated by a desire to avenge perceived slights against Islam and because of disillusionment with the wars in Iraq and Afghanistan. So far, investigators have uncovered no evidence that the bombers were connected to a terrorist group.

While explosives experts are stateside trying to answer the how of the marathon bombings, FBI agents are in the Dagestan republic of Russia -- where the brothers have roots -- trying to answer the why. Together with Russian intelligence officers, FBI agents are interviewing Zubeidat Tsarnaeva and Anzor Tsarnaev, the parents of the suspected bombers, according to the source.

Speaking in Brussels Wednesday, Secretary of State John Kerry said Tamerlan Tsarnaev went abroad "and he came back with a willingness to kill people."

Investigators are examining whether Tamerlan Tsarnaev became motivated to commit terror, in part because Russian forces cracked down on Islamic militants in Dagestan during his trip in 2012, the source said. They're also investigating whether radical fundamentalist leaders in the region may have influenced his views.

Russia contacted the U.S. government at least twice to investigate Tamerlan Tsarnaev, The Associated Press reported, concerned that he might commit a terrorist act in Russia. At the request of the Central Intelligence Agency, Tamerlan Tsarnaev's name was also added to a terrorist watchlist.

As federal investigators worked to dissect the bombs and the bombers' motives, tens of thousands of people attended a tribute in Cambridge to Massachusetts Institute of Technology Police Officer Sean Collier, 27, whom the brothers allegedly shot in cold blood three days after the bombing.

Collier's fatal shooting was the beginning of a night of violence allegedly carried out by the brothers as they sought to elude capture: Afterward they committed a carjacking, led police on a chase, engaged in a shootout with officers, hurled homemade explosives at them and grievously wounded a transit cop in the firefight, authorities have said.

Tamerlan Tsarnaev died in the firefight. His younger brother was captured later Friday, hiding in a boat in a resident's backyard. He could face the death penalty, if convicted.

Details of suspect's captureThe Associated Press reported that two U.S. officials said Dzhokhar Tsarnaev was unarmed when police captured him, bloody and injured, inside the boat.

The officials, speaking on condition of anonymity because they were not authorized to discuss the continuing investigation, told The AP that no gun was found in the boat.

Boston Police Commissioner Edward Davis said earlier that shots had been fired from inside the boat and there was an exchange of gunfire before the suspect was taken into custody.

Meanwhile, no one had yet come forward to claim the body of Tamerlan Tsarnaev from the Massachusetts Office of the Chief Medical Examiner, a spokesman for the office said Wednesday.

Zubeidat Tsarnaeva, the brothers' mother, told CNN Wednesday of her younger son: "If they are going to kill him, I don't care. My oldest one is killed, so I don't care. I don't care if my youngest one is going to be killed today. I want the world to hear this. And I don't care if I am going to get killed too."